1. Field of the Invention
The present invention relates to the field of radiant energy analyzers and more particularly, to a dual source, common mode rejection radiant energy analyzer.
2. Description of the Prior Art
Infrared or radiant energy analyzers are well known electronic systems for measurement of the absorption of energy from the radiant energy beam by a sample gas. Prior art analyzers have generally been characterized as double beam-in-space or double beam-in-time instruments. A double beam-in-space instrument is an optical system in which a reference beam is used which does not traverse the sample cell. A double beam-in-time instrument is an optical system in which the sample and reference beam, which are compared to compute the percentage concentration of a specified gas, both traverse the sample cell.
Typically, such double beam-in-time or single path dual wave length instruments involve various types of mechanical filter wheels, or rotating or reciprocating mirrors subject to mechanical failure, misalignment and degradation in performance from dirt and other substances. These susceptabilities of electromechanical infrared gas analyzers result in erroneous responses to mixtures of gaseous substances which are not uniformly distributed throughout the sample cell. In addition, the infrared analyzer itself is difficult to manufacture in large quantities. In essence, the electrooptical system and detectors of such prior systems require a large amount of bench calibration and customized adjustment.
In response to shortcomings of these prior art devices, the assignee of the present invention developed an improved infrared analyzer as described in greater detail in U.S. Pat. No. 3,745,349. In that analyzer the fine structure of the infrared absorption spectrum at a predetermined gas contained within a reference cell is compared to the fine structure of the infrared absorption spectrum obtained from the gas contained within a sample cell. An optical bench, such as shown and described below in connection with FIG. 1, is used to generate the infrared energy used for measurement and to transmit the infrared energy through the sample gas to a detector.
A primary infrared source is positioned behind the reference cell containing the reference gas. A secondary infrared source is positioned on the opposing side of the reference gas cell and between the reference gas cell and a sample gas cell. An infrared detector is then positioned on the opposing side of the sample gas cell so that the primary infrared source, the reference gas cell, the secondary infrared source, sample gas cell and the detector form a linear optical chain in that order. The infrared sources which are used are identical to each other, and are described in greater detail in U.S. Pat. No. 4,084,096 assigned to the same assignee of the present invention.
The primary and secondary infrared sources are energized in this prior art device one hundred and eighty degrees out of phase with respect to each other to effect chopped radiation transmission to the infrared detector. Energy from the primary infrared source passes through the reference cell which acts as a filter, past the secondary infrared source, through the sample cell and into the detector. Once energy from the primary source reaches the sample cell, the amount of reference gas included within the sample cell has practically no effect on the intensity of the beam from the primary source since the reference cell has filtered or absorbed substantially all of the indentifying frequencies from its spectrum. The primary energy path is referred to as the reference beam. The primary infrared source then turns off and the secondary source turns on. Energy from a secondary source passes directly through the sample cell into the detector. The amount of reference gas in a sample cell now significantly affects the energy impinging on the detector from the secondary source. The higher the concentration of the reference type of gas in the sample cell, the lesser the amount of energy from the secondary source that reaches the detector. This beam is called the sample beam.
In this type of optical bench dirt and other optical irregularities in the same cell affect or modulate the reference and sample beam to the same degree, inasmuch as the optical path, beginning at the sample cell, is identical for both the reference and sample beams. The difference in modulation between the reference beam and sample beam accounts for the analyzer's sensitivity to the reference gas.
An analog signal from the detector, which is a composite signal corresponding to the reference and sample beams is preamplified and shaped for input to a phase demodulator. During a first 180 degree phase period, the output of the demodulator alternates between an analog signal proportionate to the reference beam, and during the subsequent 180 degree phase period an analog signal proportionate to the inverse of the sample beam. When these analog signals are then integrated over time in an analog integrator circuit, the signal corresponding to the reference beam is subtracted from the signal corresponding to the sample beam and averaged over time. The output of the integrator is coupled to a servo-control circuit which is used to drive the secondary infrared source in a direction to null the output of the integrator. The amount of servo-drive used to null the integrated difference between the reference and sample beam is then read out as the relative output of the optical bench. This servo-output is then linearized and calibrated for a gain adjustment to produce an analog output representative of the percentage of reference gas in the sample cell.
While this prior art device represents a substantial improvement over infrared analyzers previously available, it still suffers from the drawback that the response times are slow and only an analog output is available. A large amount of fine tuning is required during production in order to adjust the servo-system, its calibration and linearization. The system is further characterized by a significant thermal drift or warmup period. The design is difficult to adapt to different operating formats and tends to be limited to the hardware implementation and performance of the basic design.
What is needed then is an infrared absorption analyzer which is not subject to the disadvantages of prior art analyzers as just enumerated. Such an analyzer should provide fast response times with extremely accurate digital outputs. The design should be adapted for mass production requiring little if any fine tuning or bench testing or calibration during production. The design should also be characterized by stable operation and not subject to thermal drifts or sensitivity to ambient conditions. Finally, the design must be one which is highly flexible so that the design can be easily configured in a plurality of output, control and performance configurations from the same basic design unit.